U.S. patent application number 10/037999 was filed with the patent office on 2002-12-05 for semiconductor laser device.
Invention is credited to Ito, Shigetoshi, Ono, Tomoki.
Application Number | 20020181527 10/037999 |
Document ID | / |
Family ID | 18817596 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181527 |
Kind Code |
A1 |
Ono, Tomoki ; et
al. |
December 5, 2002 |
Semiconductor laser device
Abstract
A semiconductor laser device includes an active layer and a
layer greater in bandgap energy than the active layer and having a
stacked-layer structure mainly of gallium-nitride-based
semiconductor for lasing. This device includes a
gallium-nitride-based semiconductor layer substantially equal in
bandgap to the active layer and containing at least one element
selected from the group consisting of As, P and Sb for saturable
absorption at a location apart from the active layer and inside the
layer greater in bandgap energy than the active layer.
Inventors: |
Ono, Tomoki; (Nara, JP)
; Ito, Shigetoshi; (Nara, JP) |
Correspondence
Address: |
Thomas E. Ciotti
Morrison & Foerster LLP
755 Page Mill Rd.
Palo Alto
CA
94304-1018
US
|
Family ID: |
18817596 |
Appl. No.: |
10/037999 |
Filed: |
November 9, 2001 |
Current U.S.
Class: |
372/45.013 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/3214 20130101; H01S 5/34333 20130101; H01S 5/0658
20130101 |
Class at
Publication: |
372/45 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2000 |
JP |
2000-343189(P) |
Claims
What is claimed is:
1. A semiconductor laser device having an active layer and a layer
greater in bandgap energy than said active layer, and having a
stacked-layer structure mainly of gallium-nitride-based
semiconductor for lasing, wherein a saturable absorption layer of
gallium-nitride-based semiconductor substantially equal in bandgap
to said active layer and containing at least one element selected
from the group consisting of As, P and Sb, is included in the
semiconductor laser device at a location apart from said active
layer and inside or in contact with said layer greater in bandgap
energy than said active layer.
2. The semiconductor laser device of claim 1, wherein said
saturable absorption layer forms a quantum well structure.
3. The semiconductor laser device of claim 1, wherein said
saturable absorption layer causes self-pulsation attributed to
saturable absorption effect.
4. The semiconductor laser device of claim 1, wherein said layer
greater in bandgap energy than said active layer is one of a clad
layer and an optical guide layer.
5. The semiconductor laser device of claim 1, wherein said
saturable absorption layer is formed of gallium-nitride-based
semiconductor represented by an expression
Al.sub.xIn.sub.yGa.sub.1-x-yN.sub.1-p-q-rAs.- sub.pP.sub.qSb.sub.r,
where 0.ltoreq.x, 0.ltoreq.y, x+y<1, 0.ltoreq.p, 0.ltoreq.q,
0.ltoreq.r, and 0.001.ltoreq.p+q+r.ltoreq.0.5.
6. The semiconductor laser device of claim 5, wherein in said
expression, q+r=0 and 0.005.ltoreq.p.
7. The semiconductor laser device of claim 5, wherein in said
expression, q+r=0 and 0.006.ltoreq.q.
8. The semiconductor laser device of claim 1, further comprising an
AlGaN layer covering said saturable absorption layer.
9. The semiconductor laser device of claim 1, wherein said
stacked-layer structure is formed on a GaN substrate.
10. The semiconductor laser device of claim 1, wherein said
saturable absorption layer is lower in crystalinity than said layer
greater in bandgap energy than said active layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to semiconductor laser devices
and particularly to self-pulsing, low-noise semiconductor laser
devices.
[0003] 2. Description of the Background Art
[0004] Light of a short wavelength around 400 nm will be used for a
light source in an optical disk apparatus of a next generation,
because it makes it possible to reduce a focused light beam spot
size and thus enables high-density recording. On the other hand, a
lens, an optical disk and the like are formed with inexpensive
plastic-based material to reduce their costs. Such plastic-based
material has an absorption edge wavelength of about 390 nm.
Therefore, if the wavelength of the light source is made too
shorter than 400 nm, it becomes necessary to carefully consider
material for the lens and the like and thus the too short
wavelength would not be preferable for mass production of optical
disk apparatus.
[0005] For a light source of a short wavelength of about 400 nm,
semiconductor lasers are used, which are typically formed with
gallium nitride compound semiconductor. For example, Japanese
Patent Laying-Open No. 10-294532 discloses a semiconductor laser
for an optical disk apparatus, which is formed with gallium nitride
and has a structure as shown in FIG. 11. In FIG. 11, provided over
a sapphire substrate 70 are an n-type GaN buffer layer 71, an
n-type GaN contact layer 72, an n-type AIGaN clad layer 73, an
adjacent n-type InGaN/GaN multiquantum well layer 74, an active
InGaN/GaN multiquantum well layer 75, an adjacent p-type GaN layer
76, a p-type AlGaN clad layer 77, a p-type GaN contact layer 78,
and an n-type GaN current barrier layer 79. Furthermore, a p side
electrode 80 is provided on p-type GaN contact layer 78 and an n
side electrode 81 is provided on n-type GaN contact layer 72 which
is partially exposed by anisotropic etching. In this semiconductor
laser, island regions 82 with high In concentration in adjacent
layer 74 serve as saturable absorption regions for causing
self-pulsation.
[0006] Japanese Patent Laying-Open No. 9-191160 discloses a
semiconductor laser having an InGaN saturable absorption layer,
structured as shown in FIG. 12. In FIG. 12, successively provided
over an n-type SiC substrate 60 are an n-type AIN layer 61, an
n-type AlGaN clad layer 62, an n-type GaN optical guide layer 63,
an InGaN quantum well active layer 64, a p-type GaN optical guide
layer 65, a p-type AIGaN clad layer 66, and a p-type GaN contact
layer 67. Further, an InGaN saturable absorption layer 68 is
provided inside p-type GaN optical guide layer 65. Furthermore, an
n-type electrode 59 is provided on a back surface of substrate 60
and a p-type electrode 69 is provided on p-type contact layer
67.
[0007] Conventionally in an energy band structure of InGaN used for
saturable absorption, heavy holes have large effective mass and its
valence band has an upper portion of a large state density and thus
there hardly occurs saturation of absorption regarding light from
an active layer. As such, in a semiconductor laser having a
suturable absorption layer of InGaN as described in Japanese Patent
Laying-Open No. 9-191160, saturable absorption effect hardly occurs
at low output and thus an output of a relatively high level is
required to maintain self-pulsation. Therefore, if such a
semiconductor laser is used as a light source for an optical disk,
it is disadvantageous in its power consumption and its
lifetime.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide a
semiconductor laser having a good characteristic of self-pulsation
even at low output.
[0009] To overcome the disadvantage as described above, the present
inventors considered introducing a nitride-based semiconductor
layer improved in composition for saturable absorption. As a
result, it was found that a gallium-nitride-based semiconductor
layer containing at least one element selected from the group
consisting of As, P and Sb, can be used to cause saturable
absorption and then to provide a semiconductor laser capable of
self-pulsing at low output.
[0010] Specifically, according to the present invention, a
semiconductor laser device has a stacked-layer structure mainly of
gallium-nitride-based semiconductor for laser excitation and
includes an active layer and a layer greater in bandgap energy than
the active layer, wherein the device also includes a
gallium-nitride-based semiconductor layer substantially equal in
bandgap to the active layer and containing at least one element
selected from the group consisting of As, P and Sb for saturable
absorption at a location apart from the active layer and inside or
in contact with the layer greater in bandgap energy than the active
layer.
[0011] The gallium-nitride-based semiconductor can be, for example,
GaN, Al.sub.xGa.sub.1-xN (0<x<1), In.sub.xGa.sub.1-xN
(0<x<1), In.sub.xGa.sub.yAl.sub.1-x-yN (0<x<1,
0<y<1),
Al.sub.xIn.sub.yGa.sub.1-x-yN.sub.1-p-q-rAs.sub.pP.sub.qSb.sub.r
(0.ltoreq.x, 0.ltoreq.y, x+y<1, 0.ltoreq.p, 0.ltoreq.q,
0.ltoreq.r, p+q+r<1). While the stacked-layer structure in the
semiconductor laser device of the present invention is formed
mainly with such gallium-nitride-based semiconductor as described
above, it can also include AlN, InN, InAlN and other similar III-V
compound semiconductors, particularly III-N compound
semiconductor.
[0012] In the present invention, it is preferable that the
saturable absorption layer of gallium-nitride-based semiconductor
containing at least one of As, P and Sb is formed with a quantum
well structure. Furthermore in the present invention, the layer
greater in bandgap energy than the active layer can be a clad layer
or an optical guide layer.
[0013] Typically in the present invention, the saturable absorption
layer is formed of gallium-nitride-based semiconductor represented
by an expression
Al.sub.xIn.sub.yGa.sub.1-x-yN.sub.1-p-q-rAs.sub.pP.sub.qSb.sub- .r,
wherein 0.ltoreq.x, 0.ltoreq.y, x+y<1, 0.ltoreq.p, 0.ltoreq.q,
0.ltoreq.r, and 0.001.ltoreq.p+q+r.ltoreq.0.5. In the expression,
preferably, q+r=0 and 0.005.ltoreq.p, or q+r=0 and 0.006.ltoreq.q.
This is effective as it can provide a sufficient level of
crystallinity for the saturable absorption layer.
[0014] It is preferable that the laser device of the present
invention further includes an AlGaN layer covering the saturable
absorption layer. Furthermore in the present invention, it is
preferable that the stacked-layer structure of
gallium-nitride-based semiconductor is provided on a GaN
substrate.
[0015] Furthermore in the present invention, the saturable
absorption layer can be lower in crystallinity than the layer
greater in bandgap energy than the active layer. Generally, the
saturable absorption layer is preferably grown at a lower
temperature as compared with the layer greater in bandgap energy
than the active layer.
[0016] The saturable absorption layer has, for example, a thickness
of 0.1 nm to 50 nm, preferably 0.5 nm to 20 nm.
[0017] GaNAs, GaNP, GaNSb, GaNAsP and any other similar
gallium-nitride-based semiconductor containing at least one of As,
P and Sb, hereinafter referred to as "GaN (As, P, Sb)", contains
heavy holes smaller in effective mass as compared with InGaN.
Furthermore, in the energy band structure of GaN (As, P, Sb), the
state density in the valence band is small in the vicinity of the
band end. Therefore, when light slightly greater in energy than the
substantial bandgap is absorbed, heavy hole saturation occurs more
readily in GaN (As, P, Sb) than in InGaN. As such, if a
semiconductor laser device includes a saturable absorption layer of
GaN (As, P, Sb), it can self-pulse even at low output.
[0018] Such a laser can be applied for example to an optical disk
system of low power consumption. Such an optical disk system
essentially requires a semiconductor laser not only having high
quantum efficiency but also self-pulsing even at low output and
requires a photoelectric conversion device.
[0019] Furthermore, a semiconductor laser device capable of
maintaining self-pulsation at higher output can be obtained by
providing nitride-based semiconductor layers appropriate in
structure, arrangement and number. Such a laser device allows
images, sounds and other similar information to be steadily written
in an optical disk system.
[0020] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] In the drawings:
[0022] FIG. 1 schematically shows a semiconductor laser device of a
first embodiment of the present invention;
[0023] FIG. 2 is a energy level diagram representing a
nitride-based semiconductor layer for saturable absorption;
[0024] FIG. 3 is an energy level diagram representing another
nitride-based semiconductor layer for saturable absorption;
[0025] FIG. 4 is an energy level diagram representing still another
nitride-based semiconductor layer for saturable absorption;
[0026] FIGS. 5-8 schematically show semiconductor laser devices of
second to sixth embodiments, respectively, of the present
invention;
[0027] FIG. 9 is a block diagram showing an optical information
reproduction apparatus of a seventh embodiment of the present
invention;
[0028] FIG. 10 schematically illustrates a step of providing a GaN
substrate;
[0029] FIG. 11 schematically shows a conventional semiconductor
laser;
[0030] FIG. 12 schematically shows another conventional
semiconductor laser; and
[0031] FIG. 13 is a graph showing relationship between the As
content of the InGaNAs layer in a semiconductor laser and the
self-pulsation characteristic of the laser.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0032] FIG. 1 schematically shows a semiconductor laser device in a
first embodiment of the present invention. In FIG. 1, a GaN
substrate 11 has a back surface provided with an n electrode 10 and
a front surface provided with an n-GaN buffer layer 12, an n-AlGaN
clad layer 13, an n-GaN guide layer 14, a GaNAs active layer 15, an
AlGaN ant-vaporization layer 16, a p-GaN guide layer 17, a p-AlGaN
clad layer 18, a nitride-based semiconductor layer 19, a p-AlGaN
clad layer 20, a p-GaN contact layer 21, an insulation film 22 and
a p electrode 23 successively. Furthermore, as shown in FIG. 1, a
ridge structure is formed closer to the p electrode 23 and then
electric current distribution introduced into the active layer 15
is controlled by current narrowing due to the ridge structure.
[0033] Nitride-based semiconductor layer 19 is now described more
specifically with reference to FIG. 2. FIG. 2 schematically
represents a lower limit level of a conduction band in
nitride-based semiconductor layer 19. In the figure, there is shown
a lower limit level of a conduction band in p-AlGaN clad layer 18,
a p-GaN optical guide layer 24 and a p-GaNAs monoquantum well layer
25 included in nitride-based semiconductor layer 19 for saturable
absorption, and p-AlGaN clad layer 20 in this order from the
substrate 11 side. That is, nitride-based semiconductor layer 19 is
formed of p-GaN optical guide layer 24 and p-GaNAs monoquantum well
layer 25. Nitride-based semiconductor layer 19 has a small bandgap
substantially equal to that of active layer 15 and further optical
guide layer 24 efficiently confines light emitted from the active
layer thereby to implement a structure facilitating absorption.
Hereinafter reference will be made to FIG. 1 to describe a method
of fabricating a semiconductor laser device of the first
embodiment.
[0034] A structure of a semiconductor laser device according to the
present invention is formed typically by an epitaxial growth method
to grow a crystalline film on a substrate. The epitaxial growth
method can be vapor phase epitaxy (VPE), chemical vapor deposition
(CVD), organo-metallic vapor phase epitaxy (MOVPE), organo-metallic
chemical vapor deposition (MOCVD), halide-VPE, molecular beam
epitaxy (MBE), organo-metallic molecular beam epitaxy (MOMBE),
gaseous source molecular beam epitaxy (GSMBE), chemical beam
epitaxy (CBE), or the like.
[0035] Initially GaN substrate 11 is prepared, as follows. As shown
in FIG. 10A, a SiO.sub.2 film 901 having periodic openings is
formed on a major surface of sapphire substrate 900 having a
diameter of 5.08 cm or two inches and a thickness of about 350
.mu.m. Sapphire substrate 900 is placed in an epitaxial growth
apparatus and then in a flow of H.sub.2 it is thermally cleaned at
a temperature of about 1100.degree. C. Then Ga containing gas is
prepared by supplying, to a metal Ga held at about 700.degree. C.
at a different location in the same epitaxial growth apparatus,
gaseous HCl of 100 cc per minute serving as a carrier transporting
a source material Ga and H.sub.2 of 1000 cc per minute serving as a
carrier gas. The Ga containing gas, NH.sub.3 of 2000 cc per minute
for a source material N, an n-type gaseous impurity of SiH.sub.4
and a carrier gas of H.sub.2 of 10000 cc per minute are mixed
together and supplied on sapphire substrate 900 at 1050.degree. C.
to grow a GaN monocrystalline film for three hours. A GaN
monocrystalline film thus obtained is of a thickness of about 500
.mu.m. In this process, sapphire substrate 900 has thereon
SiO.sub.2 film 901 having periodical openings so that even when the
GaN monocrystalline film having the relatively large thickness of
about 500 .mu.m grown on the sapphire substrate may not cause
cracking. Furthermore in this process, since Si is introduced
simultaneously with the crystal growth, a thick GaN monocrystailine
film of an n-conductivity type is obtained. In the present process,
such an epitaxially grown GaN buffer layer as provided in a known
epitaxy may be provided in advance between sapphire substrate 900
and SiO.sub.2 film 901. Although the fabrication process of this
method becomes somewhat complicated, a GaN monocrystalrine film of
a higher quality can be obtained.
[0036] Then a surface of the GaN monocrystalline film is lapped by
about 10 .mu.m to resolve thickness unevenness that have been
introduced during the crystal growth step. Then a grinder having a
grindstone containing diamond particles of about #400 grain size,
is used to grind sapphire substrate 900 to reduce its thickness to
about 100 .mu.m. Then a slurry containing diamond particles of
about 15 .mu.m grain diameter, is used to lap and completely remove
the sapphire base. Subsequently, the GaN monocrystailine film is
ground using a plurality of types of slurry containing diamond
particles successively reduced in diameter and finally having a
diameter of about 1/2 .mu.m. By grinding the GaN monocrystalrine
film by at least 50 .mu.m, preferably at least 100 .mu.m,
mechanical strain can be substantially removed and a region
containing many crystal defects introduced during initial crystal
growth can also be removed. Then a slurry of diamond having a
further smaller grain size is used to polish the wafer to remove
scratches therefrom to provide a mirror-finished surface.
[0037] Then to remove defects resulting from the process, an
alkaline SiO.sub.2 slurry is used to mechano-chemically polish the
wafer to obtain a surface flatten on the order of .ANG. for crystal
growth thereon. Alternatively, mechano-chemical polishing with a
KOH solution also bring about a satisfactory result. Successively
employing the both of the above mechano-chemical polishing steps
also bring about a satisfactory result. As described above, n-GaN
substrate 11 (of two inches in diameter and about 400 .mu.m in
thickness) is obtained.
[0038] Then each gallium-nitride-based semiconductor layers are
epitaxially grown on GaN substrate 11, as follows. Initially in a
MOCVD apparatus, GaN substrate 11 is set and at a relatively low
temperature of 500.degree. C. a low-temperature GaN buffer layer is
grown to a thickness of 25 nm using NH.sub.3 for a source material
for a group V element and trimethylgallium (TMGa) for a source
material for a group III element. Then at a temperature of
1050.degree. C., NH.sub.3 and TMGa plus SiH.sub.4 are introduced to
form n-GaN layer 12 (having an impurity Si content of
1.times.10.sup.18/cm.sup.3) to 3 .mu.m thickness. Then the
temperature is reduced to about 700 to 800.degree. C. and
trimethylgallium indium (TMIn) for a source material for a III
element is supplied to grow an n-In.sub.0.07Ga.sub.0.93N crack
prevention layer of 40 nm thickness. Again the substrate
temperature is increased to 1050.degree. C. and trimethylaluminum
(TMAM) for a source material for a III element is used to grow a
0.8 .mu.m thick n-Al.sub.0.1Ga.sub.0.9N clad layer 13 (having an
impurity Si content of 1.times.10.sup.18/cm.sup.- 3) and
subsequently grow n-GaN guide layer 14 to 0.1 .mu.m thickness. Then
the substrate temperature is reduced to 800.degree. C. and three
periodic GaN.sub.0.98As.sub.0.02 well layers of each 4 nm thickness
and four In.sub.0.05Ga.sub.0.95N barrier layers of each 6 nm
thickness are grown to form light emission layer 15 (having a
multiquantum well structure) by stacking a barrier layer, a well
layer, a barrier layer, a well layer, a barrier layer, a well layer
and a barrier layer in this order. The crystal growth may have an
intermission of 1 to 180 seconds between barrier and well layers or
between well and barrier layers. This allows each layer to be
flatter and a resultant laser device can emit light with a reduced
half-width.
[0039] Then the substrate temperature is increased again to
1050.degree. C. to grow 20-nm p-Al.sub.0.2Ga.sub.0.8N barrier layer
16, 0.1-.mu.m p-GaN guide layer 17 and 0.3-.mu.m
p-Al.sub.0.1Ga.sub.0.9N clad layer 18. In doing so, a p-type
impurity of Mg is added with a concentration of
5.times.10.sup.19/cm.sup.3 to 2.times.10.sup.20/cm.sup.3.
[0040] Then the substrate temperature is reduced to about
800.degree. C. to successively grow 10-nm p-GaN optical guide layer
24 and 2-nm p-GaNAs monoquantumn well layer 25 included in
nitride-based semiconductor layer 19. Monoquantumn well layer 25
has a composition of approximately GaN.sub.0.97As.sub.0.03.
Nitride-based semiconductor layer 19 formed of layers 24 and 25
contains a p-type impurity of Mg introduced therein to reduce
carrier lifetime and it is also grown at a relatively low
temperature to have crystalinity impaired as appropriate.
[0041] Again the substrate temperature is increased to 1050.degree.
C. to grow 0.2-.mu.m p-Al.sub.0.1Ga.sub.0.9N clad layer 20 and
0.1-.mu.m p-GaN contact layer 21.
[0042] As described above, TMGa, TMAl, TMIn, NH.sub.3, AsH.sub.3,
biscyclopentadienylmagnesium (CP.sub.2Mg), and SiH.sub.4 are used
as source materials for elements constituting the compound
semiconductor layers and dopants. Active layer 15 and monoquantumn
well layer 25 in nitride-based semiconductor layer 19 are formed
with 1 to 8 atomic % As set by adjusting the growth temperature,
the gas pressure, and the like. Active layer 15 and nitride-based
semiconductor layer 19 have their bandgaps adjusted to be
substantially equal to each other so that in photoluminescence (PL)
measurement, they have only a difference in PL emission peak
wavelength within .+-.20 nm (.+-.0.15 eV).
[0043] After p-GaN contact layer 21 is formed, a dry etching step
is used to form a ridge structure and then p electrode 23 of Pd/Au
is formed on an opening part of insulation film 22. Then, the back
surface of GaN substrate 11 is polished or etched to partially
remove a part of the substrate and thus adjust the thickness of the
wafer to be thin as about 100 to 150 .mu.m. This step is provided
to facilitate a subsequent step for dividing the wafer into
individual laser chips. In particular, if end face mirrors of a
laser resonator is formed when a wafer is divided into chips,
substrate 11 is preferably adjusted to have a small thickness as
about 80 to 120 .mu.m. In the present embodiment, a grinder and a
polishing machine can be used to adjust the wafer to have a
thickness of 100 .mu.m, for example. The polishing machine alone
may be used to do so. The back surface of the wafer is polished by
the polishing machine and thus flat.
[0044] After the wafer is polished, a thin metal film is deposited
on a back surface of GaN substrate 11 to obtain n electrode 10
having a Hf/Al/Mo/Au layered structure. Vacuum vapor deposition is
suitable for forming such a thin metal film with good thickness
control, and it can preferably be used in the present embodiment.
It is needless to say, however, that ion-plating, sputtering and
other similar techniques may also be used. To improve
characteristics of the p and n electrodes, formation of the metal
films is followed by annealing at 500.degree. C. to obtain good
ohmic electrodes.
[0045] A wafer with semiconductor devices fabricated as described
above is divided into chips, as follows. A diamond point is
initially used to form scribing lines on the back surface of
substrate 11 and then an appropriate level of force is applied to
the wafer to divide it along the scribing line. Alternatively, the
wafer can also similarly be divided into chips by using a wire-saw
or a thin plate blade to scratch or cut the wafer to dice it, by
using excimer laser or the like to direct a laser beam to heat the
wafer and then rapidly cooling the wafer to cause a crack as a
scribing line at the irradiated portion, or by directing a laser
beam of a high energy density to vaporize and thus abrase a portion
of the wafer to form a groove.
[0046] Then the semiconductor laser device is provided with
reflection films having a reflectance no more than 50% and that no
less that 90% at two resonator end faces respectively, and thus
asymentrically coated to obtain a steady basic transverse mode
oscillation when the device is operated at a high output of no less
than 30 mW. Then, die-bonding is employed to mount a laser chip on
a heat sink to obtain a semiconductor laser device. The heat sink
referred to herein includes a stem or the like.
[0047] Various characteristics of the semiconductor laser device
fabricated as above were estimated. When a direct current was
supplied, the semiconductor laser device started lasing at a
threshold current of 30 mA. A lasing wavelength measured with a
spectrum analyzer was 405 nm.+-.10 nm. Then the obtained
semiconductor laser device was used as a light source for an
optical disk to examine its noise characteristics for optical
feedback. It is known that continuously oscillating laser provides
an unsteady output attributed to interference with optical feedback
reflected from the optical desk. Accordingly, to enable a laser
device to have low-noise characteristics, it is desired to cause
self-pulsation at a particular cycle. The noise at an optical
output of 5 mW and optical feedback of 0.1% to 10% was found to be
no more than RIN (relative intensity noise)=-127 dB/Hz. Then, when
noise characteristics at a low output were examined, a similar
noise level of no more than -130 dB/Hz was obtained at an optical
output of about 1 mW. Thus it has been found that the semiconductor
laser device of the present invention is suitably applicable to an
optical disk system of low power consumption.
[0048] For comparison, the above described structure with
nitride-based semiconductor layer 19 replaced with an InGaN layer
was used to provide a semiconductor laser device and examine its
characteristics. The noise at an optical output of 5 mW was no more
than -125 dB/Hz, whereas that at a low output of 1 mW was no less
than -110 dB/Hz, and it was found that the laser device is not
suitably applicable to optical disk systems.
[0049] Nitride-based semiconductor layer 19 for saturable
absorption can comprise GaNAs to obtain a satisfactory
semiconductor laser, as described hereinbefore, for the following
reason. Heavy holes in GaNAs have small effective mass which is
much smaller than in InGaN in particular. When heavy holes are
light, a semiconductor comprising GaNAs has a band structure with a
valence band having an upper end in a sharp curvature, resulting in
a small state density of the valence band absorbing photons
slightly larger in energy than the bandgap. In other words, for a
large number of photons, absorption is readily saturated. A
semiconductor laser including nitride-based semiconductor layer 19
containing GaNAs causes saturable absorption even at smaller
optical output and it can thus have self-pulsation
characteristics.
[0050] The optical output causing self-pulsation can be controlled
by thickness and position in arrangement of the nitride-based
semiconductor layer for saturable absorption. Meanwhile, since an
absorption layer of a material inherently having inferior saturable
characteristic must be reduced in thickness, it would also result
in a reduced amount of light absorbed. In such case, a good
saturable absorption characteristic can hardly be obtained.
[0051] Other than the self-pulsation characteristics as described
above, a good threshold current characteristic and the like can
also be obtained by the semiconductor laser of the first
embodiment. On the other hand, if a clad layer has therein an
absorption layer of InGaN having a significantly different bandgap,
as conventional, a lattice mismatch therebetween increases lattice
defects. In particular, if a p-clad layer is impaired in
crystalinity, effect of Mg dopant would be compensated for and high
resistance would thus be caused and then lattice defects may cause
uneven current injection, impaired transverse mode oscillation, and
the like. A GaNAs layer formed inside nitride-based semiconductor
layer 19 causes the energy band to significantly bow at its small
amount of As. Thus, even when the lattice mismatch is small, a
sufficiently small bandgap can be obtained. Herein, the GaNAs layer
preferably has a bandgap width equal to or slightly smaller than
that of the active layer. From the above effects, the laser device
of the first embodiment could maintain good crystahinity of the
clad layer and achieve uniform current injection and provide
satisfactory transverse mode characteristics.
[0052] The nitride semiconductor laser structure of the first
embodiment overlies GaN substrate 11 as shown in FIG. 1.
Nitride-based semiconductor layers grown over the GaN substrate 11
have a cleavage plane matching that of the substrate and thus in
the aforementioned chip division process a good cleaved plane can
be obtained. With good resonator end faces, the semiconductor laser
device of the first embodiment can bring about end face reflection
effect as designed to cause steady self-pulsation. Furthermore, a
high yield of low-noise nitride semiconductor laser can be
obtained.
[0053] Then, nitride-based semiconductor layer 19 with GaNAs
replaced with InGaNAs was used to examine how different amounts of
As mixed affect self-pulsation characteristics. Mixing a small
amount of In, As or the like with GaN acts to reduce the bandgap.
As mentioned above, to obtain good saturable absorption, the amount
of As to be mixed was examined. As is apparent from the result
shown in FIG. 13, when As of no less than 0.5 atomic % is
contained, there can be provided a semiconductor laser device
capable of self-pulsing even at low output.
[0054] Then the ridge was altered in depth to examine how it
affects the device. As a result, although a slight change was
observed in a far filed pattern (FFP), there was not observed any
negative effect on self-pulsation characteristics. The bottom of
the ridge may have any position between p-GaN guide layer 17 and
nitride-based semiconductor layer 19.
[0055] In the FIG. 1 structure, nitride-based semiconductor layer
19 (a layer for saturable absorption) is vertically sandwiched by
p-AlGaN clad layers 18 and 20 which may have a composition
different from each other and one of which may be dispensed with.
Furthermore, the layer for saturable absorption provided inside a
p-AlGaN clad layer may alternatively be provided inside an n-AlGaN
clad layer. The two n-AlGaN clad layers vertically sandwiching the
layer for saturable absorption may be different in composition and
one of the two clad layers may be dispensed with. Furthermore, the
layer for saturable absorption may be provided inside a p- or n-GaN
guide layer. The two guide layers vertically sandwiching the layer
for saturable absorption may different in composition and one of
the two guide layers may be dispensed with.
[0056] Furthermore, if the active layer of the first embodiment
alternatively includes InGaN, InGaNAs, or InGaNAsP, setting its
bandgap to be substantially equal to GaNAs prevents saturable
absorption characteristics from varying and thus provides
satisfactory self-pulsation.
[0057] Nitride-based semiconductor layer 19 may have a structure,
such as a monoquantum well layer without an optical guide layer as
shown in FIG. 3, a multiquantum well layer 28 with an optical guide
layer 27 as shown in FIG. 4, a multiquantum well layer without an
optical guide layer, a multiquantum well layer with an optical
guide layer, or a distorted multiquantum well layer without an
optical guide layer. These structures were also expected to have a
substantially similar effect and then a lasing threshold of 30 to
50 mA and self-pulsation starting at lower output were confirmed
from results of experiments.
[0058] More specifically, FIG. 3 schematically shows a conduction
band energy structure of nitride-based semiconductor layer 19
having a configuration different from FIG. 2, illustrating the
lower limit level of the conduction band in p-AlGaN clad layer 18,
p-GaNAs monoquantum well layer 26 corresponding to nitride-based
semiconductor layer 19, and p-AlGaN clad layer 20, as seen from
substrate 11.
[0059] FIG. 4 schematically shows a conduction band energy
structure of nitride-based semiconductor layer 19 having still
another structure, illustrating the lower limit level of a
conduction band in p-AlGaN clad layer 18, p-GaN optical guide layer
27, p-GaNAs multiquantum well layer 28 and p-AlGaN clad layer 20,
as seen from GaN substrate 11. More specifically, nitride-based
semiconductor layer 19 includes p-GaN optical guide layer 27 and
p-GaNAs multiquantum well layer 28.
Second Embodiment
[0060] FIG. 5 shows a semiconductor laser device of a second
embodiment of the present invention including nitride-based
semiconductor layer 19 for saturable absorption covered with a
protection layer 29 of AlGaN. The remainder of the structure is
similar to the first embodiment. As shown in FIG. 5, n electrode 10
is provided on a back surface of 10 GaN substrate 11. Successively
provided on a front surface of GaN substrate 11 are n-GaN buffer
layer 12, n-AlGaN clad layer 13, n-GaN guide layer 14, GaNAs active
layer 15, AlGaN ant-vaporization layer 16, p-GaN guide layer 17,
p-AlGaN clad layer 18, nitride-based semiconductor layer for
saturable absorption 19, AlGaN protection layer 29, p-AlGaN clad
layer 20, p-GaN contact layer 21, insulation film 22, and p
electrode 23.
[0061] AlGaN clad layers 18 and 20 of a p type are epitaxially
grown at a high temperature. As such, it is sometimes difficult to
grow the clad layer on a thin saturable absorption containing a
precisely controlled small amount of As, P or the like without
changing the characteristics of the saturable absorption layer.
Accordingly, AlGaN protection layer 29 is formed on nitride-based
semiconductor layer 19 for saturable absorption and then p-AlGaN
clad layer 20 is formed thereover. Protection layer 29 is grown at
a lower temperature as compared with clad layer 20. Protection
layer 29 can prevent a volatile constituent of As, P or Sb from
dissipating during growth of clad layer 20 and can thus prevent
nitride-based semiconductor layer 19 from changing in composition
and characteristics.
[0062] The semiconductor laser of the second embodiment is
fabricated in a similar process as the first embodiment, except
that protection layer 29 is grown at a low temperature. The
semiconductor laser of the second embodiment thus obtained was
estimated and a result was obtained, as follows. Table 1 shows
errors between actual values of optical outputs causing good
self-pulsation and designed values thereof. It is understood from
Table 1 that protection layer 29 acts to reduce the errors. The
protection layer enables to provide a semiconductor laser device
capable of self-pulsing in a controlled condition and also serves
to increase the yield of optical disk systems which use the
semiconductor laser as a light source. The semiconductor laser of
the second embodiment has substantially similar general
characteristics of lasing threshold value, FFP and the like as the
first embodiment.
1 TABLE 1 Protection Layer Protection Layer Provided Not Provided
Offset in PL wavelength from .+-.4 nm .+-.10 nm designed value
Offset in output at which self .+-.1 mW .+-.1.5 mW pulsation starts
(at 5 mW) Yield (Reference: no protection 1.8 1 layer)
Third Embodiment
[0063] A third embodiment provides a semiconductor laser device
similar as described in the first or second embodiment, though
nitride-based semiconductor layer 19 contains GaNP rather than
GaNAs. It is fabricated in a process similar to that of the first
embodiment, except that a source material of AsH.sub.3 is replaced
with PH.sub.3. In this process, P is mixed in an amount adjusted to
be 1 to 12 atomic % in group V elements. The resultant
semiconductor laser device was evaluated regarding its
characteristics. Its lasing threshold and lasing wavelength were 34
mA and 400 nm.+-.10 nm, respectively, controlled successfully. At
an optical output of 5 mW and a quantity of optical feedback of
0.1% to 10%, its relative noise intensity was no more than -126
dB/Hz. Furthermore it was also confirmed that laser device of the
third embodiment with a protection layer similar to that in the
second embodiment serves to increase the yield of the optical desk
systems.
[0064] Self-pulsation characteristics were estimated regarding a
laser device in which InGaNP replaced GaNAs of nitride-based
semiconductor layer 19 of the first or second embodiment and it was
found that the laser device containing P of no less than 0.6 atomic
% causes good self-pulsation.
[0065] Furthermore, it was found that when the active layer of the
third embodiment alternatively comprised InGaN, InGaNAs, or
InGaNAsP, and had its bandgap substantially equal to that of GANP,
saturable absorption characteristics were not substantially changed
and good self-pulsation could be obtained.
Forth Embodiment
[0066] FIG. 6 schematically shows a semiconductor laser device in a
forth embodiment of the present invention. In FIG. 6, a GaN
substrate 111 has a back surface provided with an n electrode 110
and a front surface provided with an n-GaN buffer layer 112, an
n-AlGaN clad layer 113, an n-GaN guide layer 114, an InGaNAsP
active layer 115, an AlGaN ant-vaporization layer 116, a p-GaN
guide layer 117, a first nitride-based semiconductor layer for
saturable absorption 118, a p-GaN guide layer 119, a p-AlGaN clad
layer 120, a second nitride-based semiconductor layer for saturable
absorption 121, a p-AlGaN clad layer 122, a p-GaN contact layer
123, an insulation film 124, and a p electrode 125. As seen in FIG.
6, a ridge structure is formed closer to the p electrode to narrow
electric current distribution introduced into the active layer.
[0067] The first and second nitride-based semiconductor layers 118
and 121 are specifically shown in FIG. 2, with the conduction band
energy level having the distribution similar to that described in
the first embodiment. The first and second nitride-based
semiconductor layers 118 and 121 each include p-GaN optical guide
layer 24 and p-InGaNAsP monoquantum well layer 25 arranged
successively as seen from the GaN substrate. The first and second
nitride-based semiconductor layers 118 and 121 have their bandgaps
substantially as small as that of active layer 115 and then optical
guide layer 24 enables efficient confinement of light emitted from
active layer 115 to facilitate light absorption.
[0068] The semiconductor laser device of the fourth embodiment is
fabricated, similarly as described in the first embodiment. When
the obtained laser device received a direct current, it started
lasing at a threshold value of 35 mA. A spectrum analyzer was used
to measure the lasing wavelength, which was 405 nm.+-.10 nm.
[0069] Then the obtained semiconductor laser device was used as a
light source in an optical disk system to examine its noise
characteristics for optical feedback. It is known that continuously
oscillating laser provides an unsteady output attributed to
interference with optical feedback reflected from an optical disk.
Accordingly, to enable a laser device to have low-noise
characteristics, it is desired to cause self-pulsation having a
particular cycle. The noise at an optical output of 5 mW and
optical feedback of 0.1% to 10% was found to be no more than -130
dB/Hz. Then, the device's noise characteristics at a low output
were examined. More specifically, a similar noise level of no more
than -130 dB//Hz was obtained at an optical output of about 1 mW
and thus the laser device of the present embodiment was found
suitably applicable to optical disk systems of low power
consumption. Furthermore the device was examined for noise
characteristics at a high output. An optical output of about 30 mW
caused a similar noise level of no more than -128 dB/Hz and thus
the laser device was sufficiently useable for writing information,
such as recording images, sounds and the like, in optical disk
systems.
[0070] At low output, the semiconductor laser device internally has
a greater light distribution at the first nitride-based
semiconductor layer 118 and since the layer contains InGaNAsP
facilitating saturable absorption, i.e., the first nitride-based
semiconductor layer 118 causes saturable absorption effect, the
laser device starts self-pulsation. At high output, an effect of
carriers injected alters the carrier distribution in the
semiconductor laser device and also increases the light
distribution at the second nitride-based semiconductor layer 121.
Thus the first and second nitride-based semiconductor layers both
have saturable absorption effect contributing to self-pulsation.
The semiconductor laser device was thus capable of self-pulsing at
low output through high output.
[0071] The nitride-based semiconductor layer for saturable
absorption may includes an n-GaN optical guide layer and an
n-InGaNAs monoquantum well layer. In this case, a single
nitride-based semiconductor layer for saturable absorption may be
provided inside an n layer (e.g., an n-clad layer or an n-guide
layer) or in contact with an n-layer (e.g., an n-clad layer or an
n-guide layer) or two nitride-based semiconductor layers for
saturable absorption may be provided similarly. More specifically,
providing a layer for saturable absorption on the n layer side can
be as effective as providing it on the p layer side. Furthermore,
as described in the second embodiment, there may be included a
protection layer covering the layer for saturable absorption. The
use of the protection layer increases the yield of the laser
devices.
Fifth Embodiment
[0072] FIG. 7 schematically shows a semiconductor laser device of a
fifth embodiment of the present invention. In FIG. 7, a GaN
substrate 211 has a back surface provided with an n electrode 210
and a front surface provided with an n-GaN buffer layer 212, an
n-AlGaN clad layer 213, a first nitride-based semiconductor layer
for saturable absorption 214, an n-AlGaN clad layer 215, an n-GaN
guide layer 216, an InGaNAsP active layer 217, an AlGaN
ant-vaporization layer 218, a p-GaN guide layer 219, a p-AlGaN clad
layer 220, a second nitride-based semiconductor layer for saturable
absorption 221, a p-AlGaN clad layer 222, a p-GaN contact layer
223, an insulation film 224, and a p electrode 225. Furthermore, as
seen in FIG. 7, a stripe electrode structure is formed regarding
the p electrode to narrow electric current distribution introduced
into the active layer.
[0073] The first and second nitride-based semiconductor layers 214
and 221 for saturable absorption are specifically shown in FIG. 2,
with their conduction band energy levels having distribution
similar to that described in the first embodiment. The first and
second nitride-based semiconductor layers 214 and 221 each include
p-GaN optical guide layer 24 and p-InGaNAsP monoquantum well layer
25 arranged successively as seen from the GaN substrate. The first
and second nitride-based semiconductor layers 214 and 221 have
their bandgaps substantially as small as that of active layer 217
and then the optical guide layer enables efficient confinement of
light emitted from the active layer to facilitate light absorption.
The first and second nitride-based semiconductor layers 214 and 221
are positioned symmetrically with respect to active layer 217.
[0074] The semiconductor laser device of the fifth embodiment is
fabricated, similarly as described in the first embodiment. When
the obtained laser device received a direct current, it started
lasing at a threshold value of 32 mA. A spectrum analyzer was used
to measure the lasing wavelength, which was 405 nm.+-.10 nm.
[0075] Then the obtained semiconductor laser device was used as a
light source for an optical disk to examine its noise
characteristics for optical feedback. It is known that continuously
oscillating laser provides an unsteady output attributed to
interference with optical feedback reflected from the optical disk.
Accordingly, to enable a laser device to have low-noise
characteristics, it is desired to provide self-pulsation having a
particular cycle. The noise at an optical output of 5 mW and
optical feedback of 0.1% to 10% was found to be no more than -130
dB/Hz. Then, the device's noise characteristics at a low output
were examined. More specifically, a similar noise level of no more
than -127 dB/Hz was obtained at an optical output of about 1 mW and
thus the laser device of the present embodiment was also found
suitably applicable to optical disk systems of low power
consumption.
[0076] Furthermore the laser device of the fifth embodiment was
also examined regarding its near field pattern (NFP) and then its
beam pattern was found satisfactory, providing an improved aspect
ratio as compared with the first embodiment when beam is focused
via a lens. This improvement contributes to cost reduction as it
can reduce the number of the components of an optical system when
the laser device is applied to an optical disk system. The NFP
improved because nitride-based semiconductor layers 214 and 221 for
saturable absorption symmetrically overlying and underlying active
layer 217 enable the semiconductor laser device to internally have
symmetrical light distribution. Thus, a plurality of nitride-based
semiconductor layers of InGaNAsP for saturable absorption
symmetrically overlying and underlying an active layer enable a
semiconductor laser device to self-pulse even at low output and
also provide a superior beam pattern.
[0077] The laser device of the fifth embodiment, as well as that of
the second embodiment, may have a protection layer covering a layer
for saturable absorption. The use of the protection layer increases
the yield of the laser device. Furthermore while the fifth
embodiment employs a stripe electrode structure to narrow electric
current distribution, it may alternatively employ the ridge
structure.
Sixth Embodiment
[0078] FIG. 8 schematically shows a semiconductor laser device in a
sixth embodiment of the present invention. In FIG. 8, a GaN
substrate 301 has a back surface provided with an N electrode 300
and a front surface provided with an n-GaN buffer layer 302, an
n-AlGaN clad layer 303, an n-GaN guide layer 304, a GaNAs active
layer 305, an AlGaN ant-vaporization layer 308, a p-GaN guide layer
307, a p-AlGaN clad layer 308, an n-AlGaN block layer 309, a
nitride-based semiconductor layer for saturable absorption 310, a
p-GaN contact layer 311, and a p electrode 312. N-block layer 309
arranged inside p layer 308 narrows electric current distribution
introduced into active layer 305.
[0079] Nitride-based semiconductor layer for saturable absorption
310 is configured, as shown in FIG. 2, including p-GaN optical
guide layer 24 and p-GaNAs monoquantum well layer 25 arranged
successively, as seen from the GaN substrate. Nitride-based
semiconductor layer 310 has a bandgap substantially as small as
that of active layer 305 and then optical guide layer 24 enables
efficient confinement of light emitted from the active layer to
facilitate light absorption.
[0080] The semiconductor laser device of the sixth embodiment is
fabricated, similarly as described in the first embodiment. When
the obtained laser device received a direct current, it started
lasing at a threshold value of 25 mA. A spectrum analyzer was used
to measure the lasing wavelength, which was 405 nm.+-.10 nm.
[0081] Then the obtained semiconductor laser device was used as a
light source for an optical disk to examine its noise
characteristics for optical feedback. It is known that continuously
oscillating laser provides an unsteady output attributed to
interference with optical feedback reflected from the optical disk.
Accordingly, to enable a laser device to have low-noise
characteristics, it is desired to provide self-pulsation having a
particular cycle. The noise at an optical output of 5 mW and
optical feedback of 0.1% to 10% was found to be no more than -132
dB/Hz. Then, the device's noise characteristics at a low output
were examined. More specifically, a similar noise level of no more
than -128 dB/Hz was obtained at an optical output of about 1 mW and
the laser device of the present embodiment was also found suitably
applicable to optical disk systems of low power consumption.
[0082] The nitride-based semiconductor layer for saturable
absorption in the sixth embodiment was studied in terms of
position, structure and number and then it was found substantially
as effective as the first to forth embodiments to adopt any of a
monoquantum well layer without an optical guide layer, a
multiquantum well layer with an optical guide layer, a multiquantum
well layer without an optical guide layer, a distorted multiquantum
well layer with an optical guide layer, and a distorted
multiquantum well layer without an optical guide layer.
Furthermore, the layer for saturable absorption may be positioned
inside a p-clad layer, at a boundary of an n-block layer and p-clad
layer, inside a p- or n-guide layer, or inside an n-clad layer. Two
saturable absorption layers may be provided, as described in the
forth embodiment, enabling a nitride-based semiconductor laser
device to cause self-pulsation even at low output. Furthermore,
there may be provided a protection layer covering a saturable
absorption layer, as described in the second embodiment. The use of
the protection layer increases the yield of the laser device.
Seventh Embodiment
[0083] In a seventh embodiment, the semiconductor laser devices of
the first to sixth embodiments were evaluated regarding their
respective characteristics when they were used as a light source
for an optical disk. FIG. 9 shows an optical information
reproduction apparatus of the seventh embodiment of the present
invention. This apparatus includes a base 1001, a semiconductor
laser device 1002 arranged on the base, a collimator lens 1003, a
beam splitter 1004, an objective lens 1005, a lens 1007 condensing
reflected light, and a photodetector 1008 for detecting condensed
light.
[0084] In the FIG. 9 apparatus, semiconductor laser device 1002
outputs laser light which is in turn collimated or substantially
collimated by collimator lens 1003 and transmitted through beam
splitter 1004 and focused by objective lens 1005 on an information
recording surface of optical disk 1006. The information recording
surface has bit information written thereon by pits, magnetic
modulation, or refraction index modulation. The focused laser light
is reflected from the disk and directed through objective lens 1005
and branched by beam splitter 1004 and condensed on photodetector
1008 by lens 1007. Photodetector 1008 optically detects a signal
that is in turn converted to an electrical signal to read recorded
information.
[0085] When the laser device of the first embodiment is used in the
optical information reproduction apparatus, laser light is focused
by objective lens 1005 on the information recording surface of
optical disk 1006 with high resolution. As a result, information
written with a high density as 5M/mm.sup.2 on the optical disk
could be read with a bit error rate of 10.sup.-6. By contrast, when
such a semiconductor laser device including an InGaN saturable
absorption layer as the comparative example in the first embodiment
was used as semiconductor laser device 1002 in FIG. 9, information
was read with a bit error rate of 10.sup.-3 and the laser device
was not suitable for practical application. Thus an optical
information reproduction apparatus using the semiconductor laser
device of the present invention can read information with low noise
and hence at a low error rate from a high-density optical disk.
[0086] When the laser device of the second, third, fourth, fifth or
six embodiment instead of the first embodiment was used in the FIG.
9 apparatus, a bit error late of 10.sup.-5 to 10.sup.-7 was
achieved for any of the laser devices and it was found that the
apparatus can read information at a low noise level with the laser
device.
[0087] While mixing a small amount of As and/or P with a
nitride-based semiconductor layer for saturable absorption can
provide a laser device with satisfactory threshold characteristics
and self-pulsation characteristics, mixing Sb therewith is
similarly effective. Preferably, Sb of about 1 to 5 atomic % is
introduced into the nitride-based semiconductor layer for saturable
absorption.
[0088] Thus, in the present invention, a gallium-nitride-based
semiconductor layer containing at least one element selected from
the group consisting of As, P and Sb to cause saturable absorption,
enable to provide a semiconductor laser device capable of
self-pulsing even at low output. The semiconductor laser device of
the present invention can be operated with a power consumption
smaller than conventional and also have its lifetime longer than
conventional. The laser device of the present invention is
particularly suitable for a light source in an optical disk system.
An information reproduction apparatus using the present laser
device can read information of high density with low noise.
[0089] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *